Films of tin oxide are used on window glass to reflect infrared rays so as to keep a building cooler in the summer and warmer in the winter. For best results, the film should be relatively thick, about one-half a micron, but it is difficult to form such films that are not so rough as to diffuse light and produce an unsatisfactory hazy appearance. On the other hand, in hydrogenated amorphous silicon photovoltaic applications, a given amount of diffusion is desirable because it increases the efficiency.
The reactants from which the tin oxide film can be formed include organotins such as tetramethyl tin and tin chlorides such as dimethyl tin dichloride and tin tetrachloride also known as stannis chloride. Although tetramethyl tin more readily forms a smoother film, it yields a more limited film growth rate, and it has a dangerous subtle accumulative toxic effect on persons exposed to it. Water is not required when tetramethyl tin or dimethyl tin dichloride are used since oxygen is typically used as the oxidizing agent. However, it is required when the tin tetrachloride is used in order to obtain large growth rates below 700.degree. C.
It is known that the addition of an appropriate fluorine containing compound to the reactant gases from which tin oxide is deposited principally causes lower emissivity so as to increase reflection of infrared energy. In addition, it decreases the film roughness so as to reduce diffusion and the resulting hazy appearance. Unfortunately, however, fluorine compounds slow down the growth rate of the polycrystalline structure forming the tin oxide film making it more difficult to obtain adequately thick films.
In machines for forming a film of tin oxide on a substrate such as soda lime glass coated with silicon dioxide, the substrate is continuously moved through a deposition zone by passing it under an injector that brings the reactants in contact with the substrate by vapor deposition or by means of a spray nozzle. When tin tetrachloride is used, a desired amount of water is included in the reactants. In some machines the substrate is moved on a belt, in others on rollers, and in still others on molten tin. Exhaust ports are provided at both the upstream and downstream ends of the injector so as to remove by-products and any excess reactants. It is also customary to provide baffles adjacent the exhaust ports for reducing the turbulence in any gases that are drawn to the exhaust ports from the ends of the machine where the substrate enters and departs. In furnaces where the substrate rides on rollers or belts, these gases are ambient air, but in machines where the substrate rides on molten tin, the gases consist of a reducing atmosphere, generally nitrogen plus a small amount of hydrogen, so as to prevent the tin from being oxidized.
A number of theories have been advanced as to the reasons for the roughness of the tin oxide film, but these have overlooked an important method whereby it can be reproducibly controlled and reduced (if so desired). In the case of window glass, it would be advantageous to reduce or control the roughness, and, in the case of photovoltaic application, it would be advantageous to be able to vary the amount of roughness or haze to obtain a desired haze.
A brief description of teachings in a number of known references will now be given.
Lytle et al. No. 2,566,346 teaches a tin oxide composition useful for coating glass and other objects. The tin oxide composition and method of application provides a very high electrical conductivity for permitting the conduction of an electrical current therethrough. The coating is applied to glass heated to temperatures above 400.degree. F., but below the melting point of the glass. A solution of dissolved tin and water soluble fluoride is used in the process.
Saunders et al. No. 3,107,177 teaches a family of filming compositions and methods for producing tin oxide films that have high transparency, high electro-conductivity, and provide good electrical contact surfaces. The composition is applied to heated glass sheets.
Gillery No. 3,677,814 teaches the forming of transparent electro-conductive tin oxide films on glass via pyrolyzation of an organic tin fluoride solution having a direct tin-fluoride bond.
Terneu No. 4,293,326 teaches a process for applying a coating of tin oxide to glass. The glass is moved through coating apparatus in a zone where a gaseous medium of tin tetrachloride and water vapor contacts the glass face, and coats the same. The gas is at a temperature of about 300.degree. C., and the surface of the glass at 550.degree. C. Note in column 1 it is indicated that in the literature for prior systems "that the presence of water in the atmosphere where the coating formation occurs promotes the speed of film formation but . . . the water content of the atmosphere in which spraying takes place must be in all cases be less that 0.01 pounds per pound of air." Note in column 2, line 52 through 56 that "the present invention is based on the discovery that the strict limitation on the concentration of water vapor is not necessary provided that there is sufficient concentration of tin tetrachloride in gaseous medium from which the deposition of tin oxide occurs." Note also in column 4, line 18 that "it is preferable for the tin tetrachloride and water vapor to be supplied to the coating zone in separate gas streams so that they come into contact in the vicinity of the substrate face being coated. By this procedure premature reaction of the tin salt, causing solid deposits in the interior of a vapor feed passage can be avoided." Note in column 5, beginning on line 67 that "Nitrogen preheated to 500.degree. C. was fed from a source (not shown) along the duct 10 so as to serve as a kind of pneumatic screen isolating the passageway 13 from the gasses located in the compartment 4 and externally of the shroud 14." Note in column 6, beginning on line 10, that "The duct 12 is continuously supplied with a gas mixture at 450.degree. C. and comprising air, water, vapor and hydrofluoric acid . . . ".
Lindner No. 4,590,096 is entitled "Water Vapor, Reaction Rate and Deposition Rate Control of Tin Oxide Film by CVD on Glass". As indicated in the Abstract, the method includes providing a "gas stream containing sufficient water vapor such that its relative humidity at 18.degree. C. is about 6 percent to 100 percent. The liquid coating composition is indicated as comprising monobutyltin trichloride and trifluoroacetic acid." Note in column 5, line 26 that "Method A--dry air under controlled pressure and flow in one half inch stainless steel (SS) is passed in separate streams through a gas dryer and a water bath. The air emerging from the streams, which now contains a fixed amount of water vapor, is combined and passed through a heated oil bath and then a heated metal mixing chamber where the organotin composition is vaporized as it is injected at a controlled rate . . . ". Note further in column 5, line 43, that "Method B--Air under control pressure and flow in one half inch SS tubing is passed through a gas dryer and a water trap, combined and then passed through an in-line hygrometer. The air of known water content then passes through an electric heater into an expansion chamber one half inch in diameter. The organotin composition is injected into a nitrogen swept heated injection block at a controlled rate where it is immediately vaporized and passes into the hot-stream at the beginning of the expansion chamber. The heated mixture of air, water vapor and dopant compound now passes through a vertical tube of the same diameter enclosed in a two inch hot air insulated tube and impinges on a glass slide heated in the same manner as in Method A . . . ". In column 8, note on line 61 "Example 41" it is indicated that "The affect of Relative Humidity on deposition rate of stannic oxide was evaluated. The method of Example 1 method A and B were used to deposit the film. The results are shown in Tables IV and V. The data show that an increase in RH will generally increase the deposition rate. The relative magnitude of the increase is dependent on such parameters as additional rate and air flow." Note in column 9, line 36 "Example 45" that "the water sensitivity of the TFA dopant-MBTC system was studied using the method of Example 38. . . ".
Lindner No. 4,600,654 teaches a method for producing haze-free tin oxide coatings on glass. As indicated in column 1, beginning on line 59, "liquid monophenyltin trichloride is of low corrosivity and of low toxicity, and is rapidly decomposed in air at an elevated substrate temperature to provide a transparent, haze-free tin oxide coating . . . ". Note in column 2, beginning on line 13, the indication that "Accordingly, a carrier gas 10 which includes oxygen is metered through a feedline 11 at a predetermined flow rate through an air dryer tower 12 to provide a stream 13 of dry air. A separate airstream may be directed through a humidifier 14 containing a suitable quantity of water 15 to provide a wet airstream 16 at a desired relative humidity. Thereby in airstream 17, either dry or wet, may be passed through an evaporator 18 containing vessel 19 for holding liquid monophenyltin trichloride. The liquid is supplied to evaporator 18 by syringe pump 20 and syringe 21. The airstream is heated from an oil bath (not shown) to a desired vaporization temperature." Note also on line 47 that "The carrier gas may be dry or wet; preferably water vapor concentration is less than 10 moles of water per mole of monophenyltin trichloride."
Lindner, No. 4,737,388 teaches a system for producing transparent, non-iridescent, infrared-reflecting glass window structures which are haze-free. It does not appear that the teachings of this patent are concerned with the role of water vapor relative to hazing and coating glass sheets with tin oxide.
Lindner, No. 4,788,079 teaches a similar system to that of No. 4,737,388 for producing haze-free tin oxide coatings on glass. An air dryer tower 12 is used to provide a stream 13 of dry air. Also, a separate airstream is directed through a humidifier 14 for providing a predetermined wet airstream 16 of desired relative humidity. Either one of the dry or wet airstream 17 is passed through an evaporator 18 containing vessel 19 for holding liquid monophenyltin trichloride. Note in column 4, line 65 it is indicated that "The reason that haze-free tin oxide coatings can be produced using haze-free monophenyltin trichloride as an undercoat is not well understood at present . . . . ".
Thomas et al. No. 4,878,934 teaches an apparatus for pyrolytically forming a metal oxide coating on an upper face of a heated glass substrate. Note from the figures it appears that an injector head type coating apparatus on a continuous line is used in the process and apparatus of the invention taught. Note in column 18, line 41, Example 3, that "An aqueous solution of stannous chloride containing ammonium bifluoride was discharged at a rate of . . . ".
O'Dowd et al. 4,880,664 teaches a method for forming a textured layer of tin oxide on a vitreous substrate. It is indicated in the abstract, that the method "comprises the steps of depositing a first film of tin oxide on the substrate by chemical vapor deposition from a first reactant mixture of tin chloride, water, and an alcohol and depositing a second film of tin oxide on the first tin oxide film by chemical deposition from a second reactant mixture of tin chloride and water. Note in column 4, beginning on line 1 that "The first step includes depositing a first film of tin oxide on a substrate by chemical vapor deposition (CVD) reaction between tin chloride and water that is moderated by the addition of trace amounts of alcohol to the reaction. The second step includes depositing a second film of tin oxide on the first tin oxide film by CVD from a reactant mixture of tin chloride and water without moderation with alcohol." In column 4, line 63, it is indicated that the substrate 30 is held at a preselected reaction temperature during the deposition thereon of a chemical vapor from a first reactant mixture of tin chloride, water, and an alcohol.